Numerous pumps have already been proposed comprising at least a pump body having a chamber which communicates with an inlet duct and an outlet duct via respective inlet and outlet valves, a piston moveable in sealed manner within said chamber, a cyclical actuator such as a rotary for cam co-operating with said piston to cause it to slide, and a motor for driving the cam in an unchangeing predetermined direction while injecting volumes greater than the cylinder capacity of the pump (see for example U.S. Pat. No. 4,326,837.
Pumps of the above-specified type have already provided good service.
However, these pumps do not provide complete satisfaction, more precisely they do not provide satisfactory reproducibility when the unit volumes of sample and/or solvent to be injected are of the same order of magnitude as the cylinder capacity of the pump.
In other words, although unavoidable fluctuations in the injected volume, e.g. due to the inlet and outlet valves and/or to the compressibility of the liquids concerned, can be integrated out when the total injected volume corresponds to a large number of pump cycles, said volume fluctuations are no longer integrated out when the total injected volume is of the same order of magnitude as the cylinder capacity of the pump, e.g. when it corresponds merely to a fraction of one pump cycle.